1. Field of the Invention
The present invention relates to piezoelectric devices used as piezoelectric resonators and piezoelectric filters and to methods for manufacturing the same. In particular, the present invention relates to a piezoelectric device including a piezoelectric substrate having outer coating substrates secured on two surfaces thereof via adhesive layers, the piezoelectric substrate including an energy-trap-type piezoelectric vibration unit. The present invention also relates to a method for manufacturing such a piezoelectric device.
2. Description of the Related Art
In an energy-trap-type piezoelectric device, a piezoelectric vibration unit is provided partially on a piezoelectric substrate. The piezoelectric vibration unit must be sealed in a manner such that the piezoelectric vibration unit is not prevented from vibrating.
A piezoelectric device of the energy-trap-type described above is disclosed in Japanese Unexamined Patent Application Publication No. 8-335844. The piezoelectric device is described below with reference to FIGS. 4 and 5.
A piezoelectric device 51 shown in FIG. 4 includes a piezoelectric substrate 52 laminated with outer coating substrates 53 and 54 on the upper and lower surfaces thereof. A layered body defined by the piezoelectric substrate 52 and the outer coating substrates 53 and 54 is provided with external electrodes 55, 56, and 57.
As shown in FIG. 5, the piezoelectric substrate 52 includes driving electrodes provided partially on the upper and lower surfaces of the piezoelectric substrate 52, which define piezoelectric vibration units 52a and 52b. The outer coating substrates 53 and 54 are bonded to the piezoelectric substrate 52 via adhesive layers which have through-holes to provide spaces for the vibration of the piezoelectric vibration units 52a and 52b.
In the piezoelectric device 51, the adhesive layers include first layers 58 and 59 which are located adjacent to the outer coating substrates 53 and 54, respectively, and second layers 60 and 61 which are disposed adjacent to the piezoelectric substrate 52. The first layers 58 and 59 include a soft adhesive having a Shore D hardness of not more than 60, and the second layers 60 and 61 include a hard adhesive having a Shore D hardness of at least 60. The first layers 58 and 59 are provided with through-holes 58a and 58b and through-holes 59a and 59b, respectively. The second layers 60 and 61 are provided with through-holes 60a and through-holes 61a, respectively.
An excellent damping effect and a space for vibration are provided by the first soft adhesive layers 58 and 59 and the second hard adhesive layers 60 and 61 laminated in the piezoelectric device 51, respectively. However, when the outer coating substrates 53 and 54 expand or contract, for example, by heating when mounting on a printed circuit board, the stress produced by the expansion or contraction is absorbed by the first layers 58 and 59, thereby suppressing variation in the frequency characteristics of the device after soldering and when kept in a humid environment.
The cut-away through-holes 58b and 59b are provided at the periphery of the first layers 58 and 59 for preventing the external electrodes 55, 56, and 57 from breaking. There is a risk of breakage of the external electrodes 55, 56, and 57 in that, when applying the first soft adhesive layers 58 and 59 to the outer coating substrate 53 and 54, the adhesive tends to flow out from the periphery of the outer coating substrate 53 and 54 to the side edge portions of the layered body due to its relative softness, and the flow of adhesive to the side edges portions of the layered body prevents the external electrodes 55, 56, and 57 from being properly formed by thin-film deposition or by coating and curing a conductive paste.
In the first step of manufacturing the piezoelectric device 51, the first layer 58 of a soft adhesive is provided on a surface of the planar outer coating substrate 53, and the first layer 59 of the soft adhesive is provided on a surface of the planar outer coating substrate 54. The viscosity of an adhesive having a Shore D hardness of not more than 60 is generally not more than 1.5.times.10.sup.5 mPas at 25.degree. C. Therefore, in spite of being provided with the cut-away through-holes 58b and 59b, there is a risk of the adhesive flowing into the cut-away through-holes 58b and 59b, and flowing out of the periphery on which the external electrodes 55, 56, and 57 are provided, thereby causing a risk of breakage.
The above-described problem is eliminated when an adhesive having a Shore D harness of not more than 60 and having a viscosity before curing of at least approximately 3.0.times.10.sup.5 mPas at 25.degree. C. is used, however, such an adhesive is difficult to produce.
When manufacturing the piezoelectric device 51, the second layers 60 and 61 of a hard adhesive are provided on the first layers 58 and 59. In this case, recesses are provided in the second layers 60 and 61 in the layered body by the cut-away through-holes 58b and 59b of the first layers 58 and 59 disposed under the second layers 60 and 61. Specifically, the thickness of the adhesive varies between a region at the periphery of the layered body on which the external electrodes 55, 56, and 57 are provided and the remaining region on which the external electrodes 55, 56, and 57 are not provided, which produces gaps at the periphery of the layered body, thereby causing a risk of breakage of the external electrodes 55, 56, and 57.